1. Technical Field
The invention relates to fabrication technology for a semiconductor light-emitting device, and more particularly to a semiconductor light-emitting device with increased light extraction efficiency and the fabrication method thereof.
2. Description of the Related Art
Semiconductor light-emitting devices, such as light-emitting diodes (LEDs), have been become popular due to the small size, light weight, and low power consumption along with the increased light-emitting efficiency. Typically, an LED comprises a semiconductor p-n diode. After a bias applies to both ends of the p-n junction for passing current therethrough, electrons and holes are combined to release light. To increase LED's reliability and lower its energy consumption, increased light-emitting efficiency thereof is required.
Light-emitting efficiency of LEDs is also known as external quantum efficiency of a device, which is the product of internal quantum efficiency and light extraction efficiency of the LED. The internal quantum efficiency of the LED is the electric-optical conversion efficiency thereof determined by material properties and quality. Additionally, the light extraction efficiency is determined by the structure, the light absorption index and refractive index of LEDs. Conventionally, the internal quantum efficiency is raised by improving the quality of epitaxial layers or changing the epitaxial structure, to suppress the electric energy converting to thermal energy. Nevertheless, to further increase the light-emitting efficiency of LEDs, another important concern is how to increase light extraction efficiency thereof.
It has been proposed to roughen the profile of an LED to increase the light reflection and scattering, thereby increasing light-emitting efficiency of the LED. For example, an LED with a roughened surface can be accomplished by natural lithography, such as ion beam etching using randomly arranged polystyrene spheroids as a mask, but it may result in uneven rough surface or damage the lattice structure of the active layer of the LED and thereby reducing the light extraction efficiency of the LED. Another method for surface roughening is wet etching. In this method, an epitaxial layer for light emission is grown on a substrate with unselected lattice plane. The surface of the epitaxial layer is etched by suitable etchant with a metal mask. Since wet etching is isotropic, the epitaxial layer is etched in not only a vertical direction but also horizontal direction, so the patterns on the mask cannot effectively transfer to the surface of the epitaxial layer and thereby reducing the degree of roughening. Moreover, both ion beam etching and wet etching require an additional process step for formation of an etch mask and the process steps of both are complicated.
The surface roughening degree is proportioned to light extraction efficiency. Therefore, there is a need to develop an improved fabrication method for a semiconductor light-emitting device with higher degree of surface roughening and thereby increasing light extraction efficiency.